Despite their astonishing record of losses when dealing with lumberjacks and beavers, trees are pretty tough customers. Their trunks, branches, roots and twigs are all more than capable of enduring a winter's worth of freezing temperatures, snow, sleet and hail. Their leaves, though? Eh, not so tough. The broad, thin leaves of a broadleaf tree (like a maple, an oak, a birch, or a poplar) are an Achilles' heel when winter comes, and are vulnerable to freezing and damage from the elements. In order to survive, the trees either have to somehow protect the delicate leaves or shed them.
Evergreen trees—your pines, spruces, firs, etc.— went the protection route. Their leaves, or needles, are covered in a waxy coating to resist freezing, allowing them to live for years or even decades before falling off and being replaced. The leaves of deciduous trees, on the other hand, are cast off with the arrival of winter. The chemical processes that prepare them for their send-off also treat us to the season's vibrant colors.
Green: The green color of leaves throughout spring and summer comes from chlorophyll, a pigment vital to photosynthesis.
As we get closer to autumn and some parts of the planet get fewer hours of sunlight, trees respond by stopping the food-making photosynthesis process and slowing the production of chlorophyll until, eventually, they stop producing it altogether and the green color of the leaf fades
Yellow and Orange: Along with chlorophyll, there are yellow and orange pigments, carotene and xanthophyll, inside some trees' leaves. For most of the year, these pigments are masked by chlorophyll, but as the chlorophyll breaks down and the green color dissipates, the yellow to orange colors become visible.
Red: Another class of pigment that occurs in leaves is the anthocyanins. Anthocyanins, unlike carotene and xanthophyll, are not present in leaves year-round. It isn't until the chlorophyll begins breaking down that the plant begins to synthesize anthocyanin. Why do trees begin producing a different pigment in leaves they're getting ready to lose? The prevailing theory is that anthocyanins protect leaves from sun damage, lower their freezing point, allow them to remain on the tree longer, and buy the tree more time to recover nutrients from its leaves. The colors that anthocyanins produce are dependent on the pH of the leaves' cell sap. Very acidic sap results in a bright red color, while less acidic sap leads to a purplish red.
Brown: The humdrum color is the result of waste products trapped in the leaves.
That covers the basics of how each of the colors can be produced. But which color we ultimately see depends on several factors, such as"¦
Species: Certain colors are characteristic of particular tree species and can be used to help identify the type of tree you're looking at. Oak leaves turn red, brown, or russet, hickories turn golden bronze, poplars turn golden yellow, dogwoods turn a purplish red, beeches turn a light yellow/tan, birches turn bright yellow, sugar maples turn orange-red, black maples turn a glowing yellow, and red maples turn scarlet. Some trees, notably elms, don't go through much color change at all; there's just a dull brown and then the leaf is gone with the wind.
Weather: The temperature and moisture levels a tree is exposed to before and during the time its leaves' chlorophyll breaks down can affect color. Sunny days and cool nights favor anthocyanin production and bright red leaves. On cloudy days, anthocyanin isn't as chemically active and allows the orange or yellow pigments to take center stage.
Geography: Autumn leaves in Europe tend to be mostly yellow, but the US and East Asia seem to favor red leaves. Scientists from Israel and Finland recently put forth a theory about this color difference in the journal New Phytologist1. The scientists think that some 35 million years ago—amid a series of ice ages—many tree species evolved to become deciduous and produced red leaves to ward off insects. In North America and Asia, north-to-south mountain chains enabled the north and south spread of plants and animals corresponding with the advance and retreat of ice. In Europe, east-to-west mountain ranges like the Alps trapped plant and animal life. Many tree species (and the insects that depended on them) died out when the ice advanced. At the end of repeated ice ages, say the scientists, the tree species that survived didn't need red leaves to cope with the insects that were left, so they stopped producing red pigments and stuck with yellow.
The Dead Leaves and the Dirty Ground
While all this color changing and autumn magic is going on, the tree is preparing to cast off its leaves. Around the same time that chlorophyll production slows down, the veins that transport nutrients and water to the leaf from the rest of the tree get closed off. A layer of cells at the base of the leaf stem, called the separation layer, swells and forms a cork-like material, gradually severing the tissue that connects the leaf to the branch. The leaf falls off and the tree seals the cut—so when the leaf is blown off or falls from its own weight, a leaf scar is left behind.
1Lev-Yadun, S and Holopainen, J. (2009). Why red-dominated autumn leaves in America and yellow-dominated autumn leaves in Northern Europe? New Phytologist Volume 183(3): 506-512. doi:10.1111/j.1469-8137.2009.02904.x
Special thanks to Damian Dockery, who provided the foliage photos. See more of his work at flickr.com/damiand23.